md/raid5: If there is a spare and a want_replacement device, start replacement.

When attempting to add a spare to a RAID[456] array, also consider
adding it as a replacement for a want_replacement device.

This requires that common md code attempt hot_add even when the array
is not formally degraded.
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -7399,7 +7399,6 @@ static int remove_and_add_spares(struct mddev *mddev)
 			}
 		}
 
-	if (mddev->degraded) {
 	list_for_each_entry(rdev, &mddev->disks, same_set) {
 		if (rdev->raid_disk >= 0 &&
 		    !test_bit(In_sync, &rdev->flags) &&
@@ -7418,7 +7417,6 @@ static int remove_and_add_spares(struct mddev *mddev)
 			}
 		}
 	}
-	}
 	return spares;
 }
 

drivers/md/raid5.c

@@ -5376,8 +5376,9 @@ static int raid5_add_disk(struct mddev *mddev, struct md_rdev *rdev)
 		disk = rdev->saved_raid_disk;
 	else
 		disk = first;
-	for ( ; disk <= last ; disk++)
-		if ((p=conf->disks + disk)->rdev == NULL) {
+	for ( ; disk <= last ; disk++) {
+		p = conf->disks + disk;
+		if (p->rdev == NULL) {
 			clear_bit(In_sync, &rdev->flags);
 			rdev->raid_disk = disk;
 			err = 0;
@@ -5386,6 +5387,17 @@ static int raid5_add_disk(struct mddev *mddev, struct md_rdev *rdev)
 			rcu_assign_pointer(p->rdev, rdev);
 			break;
 		}
+		if (test_bit(WantReplacement, &p->rdev->flags) &&
+		    p->replacement == NULL) {
+			clear_bit(In_sync, &rdev->flags);
+			set_bit(Replacement, &rdev->flags);
+			rdev->raid_disk = disk;
+			err = 0;
+			conf->fullsync = 1;
+			rcu_assign_pointer(p->replacement, rdev);
+			break;
+		}
+	}
 	print_raid5_conf(conf);
 	return err;
 }